Add null check for mac_salt #787

Job	Run time
sanity-test-run	4m 54s
alpine-linux-x86 (/awslc/tests/ci/run_fips_tests.sh, --build-arg CC=clang --build-arg CXX=clang++)	32m 16s
clang-19-sanity	0s
clang-19-pedantic	0s
clang-ubuntu-2004-sanity	0s
alpine-linux-x86 (/awslc/tests/ci/run_fips_tests.sh, --build-arg CC=gcc --build-arg CXX=g++)	28m 2s
clang-ubuntu-2204-sanity	0s
clang-ubuntu-2404-sanity	0s
alpine-linux-x86 (/awslc/tests/ci/run_posix_tests.sh, --build-arg CC=clang --build-arg CXX=clang++)	1h 24m 40s
FreeBSD ${{ matrix.version }} (${{ matrix.arch }}) test	0s
alpine-linux-x86 (/awslc/tests/ci/run_posix_tests.sh, --build-arg CC=gcc --build-arg CXX=g++)	1h 4m 52s
gcc-13-pedantic	0s
gcc-4_8	6m 48s
gcc-ubuntu-2004-sanity	0s
gcc-ubuntu-2204-sanity	0s
gcc-ubuntu-2404-sanity	0s
macOS-ARM	0s
macOS-ARM-FIPS	0s
macOS-x86	0s
MSVC-2019	0s
MSVC-2022	0s
macOS-x86-FIPS	0s
MSVC-SDE-64-bit	0s
OpenBSD 7.5 (x86-64) test	44m 7s
x509-limbo tooling	0s
OpenBSD 7.6 (x86-64) test	40m 40s
OpenBSD 7.5 (arm64) test	3h 20m 0s
OpenBSD 7.6 (arm64) test	3h 20m 24s
	11h 46m 43s

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